The present invention relates to information recording media, such as optical disks, and methodology for determination of playback power in a signal playback mode. More particularly but not exclusively, this invention relates to an optical disk medium of the type permitting a playback signal to have its error rate which falls within a practically acceptable range in the case of super resolution technology being used therein. The invention also relates to a signal playback power determination method for use with such optical disks.
Information recording media typically include optical disks or discs, which are currently in widespread use. In optical disk-related technologies, a beam of laser light is focused by a lens module onto a disk for recording (writing) and reproducing (reading) data to and from the disk. Recent advances in such optical disk-related technologies enable optical disks to much increase in storage capacity. The currently available compact discs (CDs) with a diameter of 12 centimeters (cm) have a storage capacity of 650 megabytes (MB) in area density. For commercially available digital versatile disks (DVDs), about 4.7 gigabytes (GB) of storage capacity is achieved per recording layer. For HD-DVDS, 20 GB of data is recordable. For Blu-ray discs (BDs), 25 GB of data is storable. These ultralarge storage capacities of advanced optical disks have been realized mainly by shortening the wavelength λ of a laser light source from 780 nanometers (nm) to 405 nm while at the same time enlarging the numerical aperture (NA) from 0.5 up to 0.85.
For further increase in disk storage capacity, it is considered desirable to employ a method for further shortening the light source wavelength while simultaneously enlarging the lens NA value. However, an approach to further lessening the wavelength is possibly faced with difficulties in retaining high quality of record/playback signals because of the fact that the wavelength-shortened light, such as ultraviolet (UV) light, becomes more readily absorbable by a disk substrate and/or a protective film. Even when the lens NA value is made larger, a beam of near-field light is used as the data read/write light. As this near-field light is very short in distance of propagation, it is required to design optical read/write equipment so that its lens and a recording medium loaded, e.g., optical disk, are placed in very close proximity to each other. This structural configuration is much like the structure of mass-storage hard disk drive (HDD) units for use in personal computers (PCs). Thus, it is likely that it becomes more difficult in the near future to achieve easy media exchangeability which is one important feature of optical disks.
One of the presently proposed approaches to achieving ultrahigh recording densities by a different method from these methods is to use the so-called super resolution technology. This super resolution technology is the one that provides an optical disk with a certain type of mechanism for enabling successful playback of data being stored at recording marks, called the pits, with size dimensions less than or equal to the optical resolution.
A super resolution technique using a phase change film is disclosed, for example, in Japanese Journal of Applied Physics, Vol. 32, p. 5210. Typically the phase change film is used as a recording layer of several types of recordable optical disk media, such as compact disc-rewritable (CD-RW), DVD random access memory (DVD-RAM), DVD±RW and Blu-ray Disc (BD), and is made of a specific kind of material which changes in phase from a crystalline state to a fused state and also to an amorphous state in a way depending upon application of the heat generated by irradiation of laser light. In a method taught by this Japanese Journal of Applied Physics, Vol. 32, p. 5210, the phase change film is fabricated on or above a substrate of the read only memory (ROM) type. The film formed is for use as a playback layer. When a beam of laser light is irradiated onto the surface of this phase change film in a data read mode, a beam spot is formed thereon, resulting in a portion within this spot being heated and fused. In case the fused portion is higher in reflectivity than non-fused portions, the resultant playback signal becomes a signal which is significantly indicative of a signal component from the fused part. At this time, as the fused surface area is smaller than the spot, a pit signal having its resolution less than or equal to the “native” optical resolution is to be obtained as the playback signal. This kind of material that is variable in optical properties with a change in temperature, which is used for playback of the data being stored at the pits of less than the optical resolution, is called the super resolution material.
Another super resolution technique using a phase change film made of the super resolution material is found in Japanese Journal of Applied Physics, Vol. 43, p. 4921. In a method as disclosed in this paper, an optical disk of the type having both a platinum oxide film and a phase-change recording film is used. A pulse laser beam is irradiated onto the disk to form therein recording marks for data recording in a way similar to mark creation processes in ordinary recordable optical disks, which marks will be read by super resolution playback, thereby improving the recording density. With this method, by irradiation of incoming write laser power, the platinum oxide film is locally expanded, resulting in the thickness of the phase change film being modulated in a way corresponding to the marks created. In a playback mode, only a thin portion of the thickness of the phase change film is fused to thereby obtain the super resolution effect. This enables achievement of a one-time recordable super resolution optical disk, also known as write-once-read-many (WORM) disk.
Still another super resolution technique using a phase change film as the super resolution material is suggested in Japanese Journal of Applied Physics, Vol. 45, p. 2593. In a method disclosed therein, an optical disk is used which is structured so that only pits contain the phase change material and each pit is interposed between adjacent gap spaces, like an isolated island. When reading this disk, the phase change film is partly fused by irradiation of laser light at a portion within the pit of interest so that this pit changes in reflectivity. When this change causes a playback signal to increase in magnitude, a large amount of playback signal is obtained from the pit that is in the fused area within a beam spot. Thus, the intended super resolution is realized. With this method, since the pit is isolated by spaces, it is possible to limit the fused area more successfully than the case of the phase change film being used for an entire disk surface. It is also possible to lessen thermal conduction to its neighboring pits. Accordingly, the use of this method makes it possible to play back the data being stored at ultrasmall sizes of pits.
A multilayer disk design technique is also proposed as one method of achieving higher recording densities in a way different from the techniques disclosed in the above-identified three Japanese technical bulletins. In this multilayer technique, an optical disk is used which is arranged to have a multilayer structure of spaced-apart data layers. Record/playback operations are performed in a way independent of each other while letting a beam of laser light be focused on each layer. Thus, it can be said that the multilayer disk technique is the method for increasing the storage capacity of an optical disk in the direction of its thickness or “volume” in a certain sense.